1. FIELD OF THE INVENTION
The present invention relates to a method for washing an organic solution of a polycarbonate (hereinafter referring to as polycarbonate solution). More particularly, it relates to a method for washing the polycarbonate solution containing an impurity, such as a crude polycarbonate solution produced by the phosgene method, with an aqueous washing liquid.
2. DESCRIPTION OF THE PRIOR ART
The polycarbonate solution obtained from a reaction system using the phosgene method contains impurities such as an alkali metal salt of the unreacted dihydroxy compound, residual amines used as the catalyst, alkali metal halides, alkali metal hydroxides and alkali metal carbonates. These impurities are incorporated with a small amount of water in the form of an emulsion. When the impurities remain in the product, a coloring of the product is disadvantageously caused. It is necessary to remove these impurities from the polycarbonate solution as described above.
In order to remove these impurities from the resulting polycarbonate solution, it is customary to carry out, in sequence, the following purification.
(1) Water washing to remove various impurities such as the alkali metal salt of the unreacted dihydroxy compound and other water soluble impurities.
(2) Washing with an acidic aqueous solution for neutralization of residual base or separation of amines used as the catalyst in a polymerization reaction.
(3) Water washing to remove excess acid from the acidic bath and water soluble impurities.
Thus, the impurities in the polycarbonate solution are found dissolved in the water incorporated in the water in oil dispersed emulsion.
As the emulsion dissolved impurities are relatively stable under alkaline conditions, the water washing in the washing operation (1) which is carried out under alkaline conditions, is not easy to carry out effectively. It has been especially difficult under industrial conditions to wash polycarbonate solutions having a high concentration of polycarbonate and high viscosity.
It has also been known to add a small amount of an aqueous washing liquid to the polycarbonate solution to disperse the aqueous washing liquid in the polycarbonate solution in the form of a water in oil dispersed phase. According to this method, the impurities incorporated in the polycarbonate solution in the form of an emulsion are dissolved in the aqueous washing liquid whereby excellent washing can be expected.
However, the time for separating the aqueous washing liquid in the form of water in oil dispersed phase from the polycarbonate solution by this method is too long to be suitable for an industrial operation.
It has further been known to wash the polycarbonate solution with a large amount of aqueous washing liquid.
In said case, the polycarbonate solution is dispersed in the aqueous washing liquid in the form of an oil in water dispersed phase. It is easy to separate the aqueous washing liquid from the polycarbonate solution. However, elution of the impurities in the form of an emulsion into the aqueous washing liquid is remarkably slow whereby it is difficult to attain satisfactory washing effect.
The inventors have found a more effective method for washing the polycarbonate solution. As the result, the inventors have found that the impurities can be effectively removed from the polycarbonate solution by the combination of certain steps.
In accordance with this method, satisfactory washing can be performed even though the concentration and viscosity of the polycarbonate solution are high or it is alkaline. Moreover, a satisfactory result can be attained even through the use of a simple apparatus such as a combination of a stirring vessel and a settler separator. The present invention has been attained by the following procedures.